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Photodetectors

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Avalanche ... field result in more ionization called avalanche effect. ... light. Temperature effect on avalanche gain. Comparison of photodetectors ... – PowerPoint PPT presentation

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Title: Photodetectors


1
Chapter 6
  • Photodetectors

2
Content
  • Physical Principles of Photodiodes
  • pin, APD
  • Photodetectors characteristics (Quantum
    efficiency, Responsivity, S/N)
  • Noise in Photodetector Circuits
  • Photodiode Response Time
  • Photodiodes structures

3
pin Photodetector
w
The high electric field present in the depletion
region causes photo-generated carriers to
Separate and be collected across the reverse
biased junction. This give rise to a current
Flow in an external circuit, known as
photocurrent.
4
Energy-Band diagram for a pin photodiode
5
Photocurrent
  • Optical power absorbed, in the depletion
    region can be written in terms of incident
    optical power,
  • Absorption coefficient strongly depends
    on wavelength. The upper wavelength cutoff for
    any semiconductor can be determined by its energy
    gap as follows
  • Taking entrance face reflectivity into
    consideration, the absorbed power in the width of
    depletion region, w, becomes

6-1
6-2
6
Optical Absorption Coefficient
7
Responsivity
  • The primary photocurrent resulting from
    absorption is
  • Quantum Efficiency
  • Responsivity

6-3
6-4
6-5
8
Responsivity vs. wavelength
9
Avalanche Photodiode (APD)
APDs internally multiply the primary photocurrent
before it enters to following circuitry. In
order to carrier multiplication take place, the
photogenerated carriers must traverse along a
high field region. In this region, photogenerated
electrons and holes gain enough energy to ionize
bound electrons in VB upon colliding with them.
This multiplication is known as impact
ionization. The newly created carriers in the
presence of high electric field result in more
ionization called avalanche effect.
Optical radiation
Reach-Through APD structure (RAPD) showing the
electric fields in depletion region and
multiplication region.
10
Responsivity of APD
  • The multiplication factor (current gain) M for
    all carriers generated in the photodiode is
    defined as
  • Where is the average value of the total
    multiplied output current is the primary
    photocurrent.
  • The responsivity of APD can be calculated by
    considering the current gain as

6-6
6-7
11
Current gain (M) vs. Voltage for different
optical wavelengths
12
Photodetector Noise S/N
  • Detection of weak optical signal requires that
    the photodetector and its following amplification
    circuitry be optimized for a desired
    signal-to-noise ratio.
  • It is the noise current which determines the
    minimum optical power level that can be detected.
    This minimum detectable optical power defines the
    sensitivity of photodetector. That is the optical
    power that generates a photocurrent with the
    amplitude equal to that of the total noise
    current (S/N1)

13
Signal Calculation
  • Consider the modulated optical power signal P(t)
    falls on the photodetector with the form of
  • Where s(t) is message electrical signal and m is
    modulation index. Therefore the primary
    photocurrent is (for pin photodiode M1)
  • The root mean square signal current is then

6-8
6-9
6-9
6-10
14
Noise Sources in Photodetecors
  • The principal noises associated with
    photodetectors are
  • 1- Quantum (Shot) noise arises from
    statistical nature of the production and
    collection of photo-generated electrons upon
    optical illumination. It has been shown that the
    statistics follow a Poisson process.
  • 2- Dark current noise is the current that
    continues to flow through the bias circuit in the
    absence of the light. This is the combination of
    bulk dark current, which is due to thermally
    generated e and h in the pn junction, and the
    surface dark current, due to surface defects,
    bias voltage and surface area.
  • In order to calculate the total noise presented
    in photodetector, we should sum up the root mean
    square of each noise current by assuming that
    those are uncorrelated.
  • Total photodetector noise currentquantum noise
    current bulk dark current noise surface
    current noise

15
Noise calculation (1)
  • Quantum noise current (lower limit on the
    sensitivity)
  • B Bandwidth, F(M) is the noise figure and
    generally is
  • Bulk dark current noise
  • is bulk dark current
  • Surface dark current noise is the
    surface current.

6-11
6-12
Note that for pin photodiode
6-13
16
Noise calculation (2)
  • The total rms photodetector noise current is
  • The thermal noise of amplifier connected to the
    photodetector is
  • input resistance of amplifier, and
    is Boltzmann cte.

6-14
6-15
17
S/N Calculation
  • Having obtained the signal and total noise, the
    signal-to-noise-ratio can be written as
  • Since the noise figure F(M) increases with M,
    there always exists an optimum value of M that
    maximizes the S/N. For sinusoidally modulated
    signal with m1 and

6-16
6-17
18
Photodetector Response Time
  • The response time of a photodetector with its
    output circuit depends mainly on the following
    three factors
  • 1- The transit time of the photocarriers in
    the depletion region. The transit time
    depends on the carrier drift velocity and
    the depletion layer width w, and is given by
  • 2- Diffusion time of photocarriers outside
    depletion region.
  • 3- RC time constant of the circuit. The
    circuit after the photodetector acts like RC low
    pass filter with a passband given by

6-18
6-19
19
Photodiode response to optical pulse
Typical response time of the photodiode that is
not fully depleted
20
Various optical responses of photodetectors
Trade-off between quantum efficiency response
time
  • To achieve a high quantum efficiency, the
    depletion layer width must be larger than
  • (the inverse of the absorption
    coefficient), so that most of the light will be
    absorbed. At the same time with large width, the
    capacitance is small and RC time constant getting
    smaller, leading to faster response, but wide
    width results in larger transit time in the
    depletion region. Therefore there is a trade-off
    between width and QE. It is shown that the best
    is

21
Structures for InGaAs APDs
  • Separate-absorption-and multiplication (SAM) APD
  • InGaAs APD superlattice structure (The
    multiplication region is composed of several
    layers of InAlGaAs quantum wells separated by
    InAlAs barrier layers.

light
22
Temperature effect on avalanche gain
23
Comparison of photodetectors
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